The present invention relates to an alternator for an automotive vehicle, which is preferably installed in a passenger vehicle, a truck or the like.
Vehicle bodies tend to be formed into slanted nose shapes in order to reduce their aerodynamic resistance. However, a vehicle body is also required to provide enough space for the passenger compartment. To satisfy both requirements, the engine compartments of automotive vehicles have become so narrow and crowded that only limited space is available for installing an alternator. Meanwhile, to improve fuel economy, the rotation rate of the engine tends to be reduced. Correspondingly, the rotation rate of the alternator is lowered. At the same time, however, there is a need to increase electric loads such as safety control devices, etc. Thus, improving the generating ability of the alternator is strongly required. In other words, a compact, powerful, and inexpensive automotive alternator is needed.
Furthermore, it is desirable that noises leaking from vehicles be reduced. Providing a quiet passenger compartment leads to increased product appeal. To this end, engine noises have recently been reduced. However, engine accessories need to rotate at relatively high speeds. Thus, fan noises and magnetic noises of automotive alternators are becoming the main noises emitted from automotive vehicles.
Conventionally, the stator winding generally used in an automotive alternator is arranged by using a continuous wire assembled with a stator core. In such a stator winding arrangement, various improvements have been proposed for satisfying the requirements of compactness, higher output, lower noise, etc.
For example, as disclosed in Published Unexamined Japanese Patent Application No. 7-303351, providing a shorter winding is effective to reduce the winding resistance. According to a 2.pi./3 short-pitch winding technique, the winding is wound without causing interference between phases in a radial direction. However, there is a problem in that the winding factor is substantially worsened and accordingly the generated voltage is greatly reduced. Furthermore, performing such a winding operation is difficult.
According to other proposed techniques, coil ends are pre-shaped or thinned to avoid interference during the winding operation. However, problems arise in that the winding operation is complicated and the winding resistance value is increased. Furthermore, according to these techniques, the interference of the coil ends cannot be solved completely.
In each slot, the coil is located in an offset condition where no more than half of a cross section is available as a geometrical space for accommodating the coil. This arrangement results in a drawback because the resistance cannot be reduced. Also, due to the above-described offset arrangement of the coil in the slot, the coil configuration becomes different in each phase, making the resistance value and the inductance of the winding different in each phase. Thus, the flow of current is different in each phase, causing a local temperature increase which may worsen the performance of the alternator and increase the magnetic noise.
As disclosed in Published Unexamined Japanese Patent Application No. 59-159638, the coil end may be formed into a flattened configuration to reduce the air flow resistance. However, according to the arrangement shown in this prior art, the air flow resistance is still high. Sufficient cooling ability cannot be expected, and noise cannot be reduced satisfactorily.
Furthermore, to realize compactness and high output, there is a technique for increasing the magnetic flux by reducing the air gap between the rotor and the stator. However, this technique requires enlarging the cross-sectional area of the stator core in accordance with an increase of the magnetic flux, thereby reducing the slot area and increasing the winding resistance. As a result, the effect of increased output is largely canceled. In short, it is important to optimize the balance between the core and the winding which cooperatively constitute the stator.
To obtain an improved output, an optimization will be attained in the selection of design data for the core cross section and the winding. However, a remaining problem to be solved is cooling of the coil ends which serve as a heat generation source. For example, a large-scale fan will be required for cooling the electric conductors through an insulating film and a fixing member provided on the surface thereof. The fan needs to be disposed adjacent to the coil ends. However, according to a conventional winding, the coil ends are undulated due to the interference between different phases. This increases the higher-order fan noise. When this noise creates a nuisance for vehicle passengers, it must be reduced. For example, the inner face of the coil ends facing the fan may be smoothed as an ideal surface by using a complicated winding method, or the cooling air amount may be reduced by sacrificing fan efficiency.
In the process of seeking compactness and higher output, there will be a problem in that the magnetic noise is increased due to an increased magnetic force acting between the rotor and the stator. In general, an automotive alternator is equipped with a rectifier which cuts the output voltage at a predetermined level to charge a battery. Thus, the generated voltage has a rectangular waveform. It is known that spatial higher harmonics in the clearance between the stator and the rotor chiefly comprise tertiary higher harmonic components. Accordingly, a magnetic force acting between the stator and the rotor comprises the square components of the tertiary higher harmonic components. The magnetic force thus created will generate a magnetic ripple force.
To eliminate such magnetic forces, Published Unexamined Japanese Patent Application No. 4-26345 proposes to use two sets of three-phase windings which are mutually phase shifted by an electric angle of 30.degree.. Two outputs of these two three-phase windings are combined so that their magnetic ripple forces will cancel each other. However, this conventional winding arrangement cannot solve the coil end interference derived from the conventional winding configuration. The number of required slots is doubled, which requires a careful winding operation for a thin wire wound in the slots. This problem is difficult to solve. Clearly, when compactness and higher output are the object, there are many new problems to be solved.
A stator winding having a continuous wire is widely used in conventional automotive alternators. However, this type of stator winding cannot satisfy all of the requirements above, such as compactness, higher output and lower noise, which are contradictory to each other.
On the other hand, a general large generator, such as an induction-type generator, may comprise two layered conductors accommodated in a stator slot which constitute inner and outer layers disposed in a radial direction of the stator slot. To eliminate the interference between different phases at the coil ends, the conductors of the inner and outer layers are alternately connected.
However, there was a problem in that the above-described larger generator could not be directly used as an automotive alternator. More specifically, the automotive alternator needs to supply electric power to the automotive electric loads during an engine idling condition corresponding to the lowest engine speed region and equivalent to the alternator's rotation range of approximately 1,500 rpm. To this end, it is necessary to generate approximately 15 V, which is equivalent to a sum of a battery voltage and a diode drop, at the above-described rotational speed range, i.e., approximately 1,500 rpm or less. However, for an automotive alternator of 1 to 2 kW used for a general passenger vehicle or a truck, it is difficult to generate 15 V at such a low rotation. The above-described general large induction-type generator has about two conductors per slot which is chiefly dependent on a magnetic flux determined by the physical size. When the number of conductors is as small as two, it is difficult to supply sufficient electric power because an electromotive voltage cannot be obtained satisfactorily at the above-described low rotation region. Furthermore, there is a recent tendency to reduce the idling rotation further in order to improve fuel economy. Responding to such a requirement is difficult for the above-described general large induction-type generator.
To increase output at a low-speed region, it may be possible to employ a multi-polarized arrangement operated by higher frequencies. Toward this end, the above-described general large-scale generator uses a salient rotor having an axial length substantially equal to that of the stator core. According to the salient rotor, the magnetomotive force of each magnetic pole is reduced in response to an increase of the magnetic pole number because of a reduction of wiring space in the rotor. Thus, it is difficult to increase the output. In other words, the above-described automotive alternator cannot attain the required performance.
In general, providing clearance inside the salient rotor is structurally difficult. Thus, there is a problem in that the cooling air cannot be introduced and guided to an inner peripheral surface of the rotor. Furthermore, the cooling air cannot be introduced and guided to a field coil provided in the rotor.
Published Unexamined Japanese Patent Applications No. 62-272836, No. 63-274335 and No. 64-5340 disclose automotive alternators which use U-shaped electric conductors, i.e., so-called conductor bars. However, according to this prior art, a plurality of stator cores are laminated in a circumferential direction so as to form a cylindrical shape. Thus, magnetic resistance increases with respect to the passing direction of the magnetic flux, and the required performance cannot be realized. There are also many problems to be solved, for example, maintaining the practical strength.
PCT document WO 92/06527 proposes an arrangement of an automotive alternator which uses conductor bars for a stator. According to the arrangement shown in this prior art, a total of four electric conductors are disposed in a square shape in each slot. In a slot, clearance is provided between two electric conductors arrayed in a circumferential direction at a coil end. According to a conventional ventilation structure for an automotive alternator, a cooling fan is provided outside a frame, and a ventilation passage is provided for guiding cooling air in the axial direction. On the contrary, according to applicant's ventilation structure, a cooling fan is located inside a frame so that cooling air can be directly supplied to the coil end. With this arrangement, the cooling ability is greatly improved so that compactness and higher output can be realized. Accordingly, providing a clearance between coil ends makes it possible to reduce the air resistance and increase the cooling ability. Furthermore, using the U-shaped conductor bars makes it easy to insert the conductor bars into slots offset by a magnetic pole pitch.
However, according to this arrangement, the cross section of each electric conductor is limited. If the cross section of each electric conductor is increased to reduce the electric resistance and increase the output, then it will not be possible to provide the above-described clearance, which will significantly worsen the cooling ability and formability at the coil end. To secure sufficient clearance, it may be effective to reduce the number of electric conductors from four to two per slot. However, with such a small number of conductors, the device will not be able to generate an output at an idling rotation, i.e., at a low-speed region, and thus cannot be used as an automotive alternator.
U.S. Pat. No. 2,928,963 proposes an AC generator which comprises a stator using conductor bars and a Lundel-type pole core. However, according to this invention, a ventilation structure extends only in an axial direction. This ventilation structure differs from the present general ventilation structure. Furthermore, even in an arrangement shown as another embodiment, no internal fan is provided. In view of the foregoing, it is concluded that no improvement is shown for increasing the cooling ability to realize both compactness and higher output. Furthermore, according to the arrangement disclosed in this prior art, each slot accommodates two conductors. Thus, as described above, it will be difficult to obtain an output in the lower-speed region with such a device.